The invention relates generally to systems and methods for containing fluids expelled from a subsea wellhead. More particularly, the invention relates to remedial systems and methods for containing fluids discharged from the cement ports of a subsea wellhead.
In offshore drilling operations, a large diameter hole is drilled to a selected depth in the sea bed. Then, a primary conductor secured to the lower end of an outer wellhead housing, also referred to as a low pressure housing, is run into the borehole with the outer wellhead housing positioned at the sea floor. A wellhead guide base used to facilitate subsequent installation of equipment is typically mounted to and run with the outer wellhead housing. Cement is pumped down the primary conductor and allowed to flow back up the annulus between the primary conductor and the borehole sidewall.
With the primary conductor secured in place, a drill bit is lowered through the primary conductor to drill the borehole to a second depth. Next, an inner wellhead housing, also referred to as a high pressure housing, is seated in the upper end of the outer wellhead housing. A string of casing secured to the lower end of the inner wellhead housing or seated in the inner wellhead housing extends downward through the primary conductor. Cement is pumped down the casing string, and allowed to flow back up the annulus between the casing string and the primary conductor and out cement ports extending radially through the outer wellhead housing. The cement ports can be opened to allow flow therethrough, or closed to prevent flow therethrough, by a cement port closure sleeve moveably disposed over the cement ports. Drilling continues while successively installing concentric casing strings that line the borehole. Each casing string is cemented in place by pumping cement down the casing and allowing it to flow back up the annulus between the casing string and the borehole sidewall.
Following drilling operations, the cased well is converted for production by running production tubing through the casing, which is typically suspended by a tubing hanger seated in a mating profile in the inner wellhead housing. A production tree having a production bore and associated valves is lowered subsea and mounted to the inner wellhead housing.
The failure of seals between the inner wellhead housing or casing and the outer wellhead housing or primary conductor, and/or failure of the cement port closure sleeve may result in leakage of fluid trapped in the annulus between the inner wellhead housing or casing and the outer wellhead housing or primary conductor. Such fluids may include drilling mud trapped in the annulus during drilling of the well. In instances where oil based muds were used to drill the borehole, leakage of drilling mud from the annulus into the surrounding sea water is particularly problematic from an environmental regulations perspective. For example, FIGS. 1 and 2 illustrate a subsea well 10 extending downward from the sea floor 11. Well 10 includes an outer wellhead housing 20 proximal the sea floor 11, a primary conductor 21 extending downward from outer wellhead housing 20, a wellhead guide base 22 mounted to outer wellhead housing 20, an inner wellhead housing 23 seated in outer wellhead housing 20, a casing string 24 extending downward from inner wellhead housing 23, and a production tree 25 coupled to inner wellhead housing 23. An annulus 26 is formed between casing string 24 and primary conductor 21. Outer wellhead housing 23 includes cement ports 27 extending radially therethrough and a cement port closure sleeve 28 for closing off ports 27. Normally, annulus 26 is filled with cement. However, in some cases, drilling fluids may get trapped within the upper portion of annulus 26 proximal ports 27. If sleeve 28 is unable to fully close ports 27 (e.g., due to failure of a seal, etc.), such drilling fluids may undesirable leak from well 10 into the surrounding sea water.